1. Field of Invention
The present invention relates to semiconductor memories and in particular to write error protection.
2. Description of Related Art
As semiconductor product is made from very deep sub micron (VDSM) semiconductor circuitry coupling capacitance between wires has an increasing effect on circuit operations. This is particularly true in semiconductor memories where interconnections, such as bit lines, are often close together and run in parallel for a considerable distance. A data error write can occur, for instance, in an SRAM as a result of coupling capacitance between bit lines, and occurs on memory cells in adjacent columns to the memory column being selected.
In U.S. Pat. No. 4,623,989 (Blake) an SRAM is directed to cells that have P-channel access and driver transistors. Bit lines in the SWAM are precharged to a voltage close to Vss and wordlines are held near Vcc in the off state in order to have good immunity to read and alpha particle induced errors. In U.S. Pat. No. 4,618,945 (Sukurai et al.) is directed to a memory device with an array of memory cells. Rows of memory cells are connected by wordlines which include a first and second wordline. In response to a column address signal a switch in each row is turned on that connects a first wordline to a second wordline. In U.S. Pat. No. 4,586,166 (Shah) is directed to an SRAM where positive feedback is used with bit line loads during read and write operations, avoids read after read errors, and having a short write time.
In FIG. 1a is shown a six transistor memory cell where transistors Qb, Qd, Qe and Qf are cross coupled to produce a bistable circuit with stored voltage at DL and DLB. Transistors Qa and Qc controlled by a wordline WL connect the stored voltage at DL to bit line BL and stored voltage at DLB to bit line BLB. In FIG. 1b is shown a four transistor memory cell. Transistors Qb and Qc are cross coupled to produce a bistable circuit with stored voltages at DL, and DLB. Resistors R1 and R2 connect Vdd to transistors Qb and Qd respectively. Transistors Qa and Qc controlled by a wordline WL connect the stored voltage at DL to bit line BL and stored voltage at DLB to bit line BLB.
In FIG. 2 is shown a portion of an SRAM memory array of prior art. Memory cells represented by MC22 are connected in columns to bit lines represented by bit lines BL2 and BL2B, where BL2B is the logical compliment of BL2. Adjacent to the column containing memory cell MC22 is a column to the left containing a memory cell MC11 and a column to the right containing memory cell MC31. Each bit line is connected to bit line drivers 10 which are used to read and write the data contained within each cell under the control of the Y decoder 11. A first wordline WL1 connects to one cell in each column MC11, MC21, MC31 and MCn1. A second wordline WL2 connects to the row of memory cells containing MC12, MC22, MC32 and MCn2, and a third wordline WL3 connects to a row on memory cells containing MC13, MC23, MC33 and MCn3. Each row of memory cell is connected to a wordline similar to wordline WL4 connected to a row of memory cells containing memory cell MC24.
Continuing to refer to FIG. 2, since the memory array is organized into rows and columns, the bit lines of adjacent columns are routed in parallel with each other. This results in coupling capacitance such as C1 between BL1 and BL1B, C2 between BL1B and BL2, C3 between BL2 and BL2B, C4 between BL2B and BL3, C5 between BL3 and BL3B and so on. As semiconductor devices are made smaller, the columns become closer together and the coupling capacitance becomes larger allowing more energy to be coupled between bit lines to a point where a write disturb condition results at memory cells in adjacent columns to the memory column where a write operation is being performed.
Continuing to refer to FIG. 2 along with FIGS. 1a and 1b, if memory cell MC22 is to be written, wordline WL2=1, all other word lines are held at ground potential, and the Y decoder 11 activates the bit line drivers for the column to which data is to be written. Assuming that the initial logic states DL of MC12, MC22, MC31 and MC33 are high and logic states at DLB of MC12, MC22, MC31 and MC33 are low; logic states at DL of MC11, MC13 and MC32 are low and logic states at DLB of MC11, MC13 and MC32 are high. If the previous data written into MC22 is BL2=high and BL2B=low, then in the next cycle when the new data makes BL2=low and BL2B=high a voltage change takes place on bit line BL2 that is coupled into bit line BL1B, and similarly a voltage change takes place on bit line 13L2B that is coupled into bit line BL3. If a memory cell such as MC11 and MC13 that are not connected to WL2 has data stored and BL1 is in a low state (ground), then a negative voltage coupled into bit line BL1B can be sufficient to turn on transistor Qc (shown in FIGS. 1a and 1b) overwrite the data in cells MC11 or MC13, and destroy the stored data. In like manner, again in the next cycle, when the new data makes BL2=high and BL2B=low, a voltage change takes place on bit line BL2B that is coupled into bit line BL3, and similarly a voltage change takes place on bit line BL2 that is coupled into bit line BL1B. If a memory cell such as MC31 and MC33 that are not connected to WL2 has data stored and BL3 is in a low state (ground), then a negative voltage coupled into bit line BL3 can be sufficient to turn on Qc (show in FIGS. 1a and 1b) to overwrite the data in cells MC31 or MC33, and destroy the stored.
In FIG. 3 is shown a diagram of prior art of an SRAM with bit line precharge circuitry used to improve error protection during write operation. Precharge circuits 12 are connected to each bit line BL1, B12, BL3, and BLn, and bit line bar B11B, BL2B, BL3B and BLnB. The precharge circuits 12 are controlled from a precharge control circuit 13. The precharge circuits 12 precharge the bit lines to a voltage less than VDD by an amount approximately equal to a transistor threshold. The precharge control 13 is activated using chip enable CE and write enable WE. Each time a write operation is done all bit lines in FIG. 3 are precharged to prevent voltage coupled from adjacent bit lines from destroying data in memory cells not being written. The disadvantage with this approach is that there is a considerable amount of power dissipated to provide precharging to every bit line during a write operation to a cell in a column when only bit lines adjacent to those involved in the write operation are at risk for coupling a charge that can destroy data in a memory cell no being written.
An objective of the present invention is to prevent disturb conditions resulting from a write operation from destroying data stored in a memory cell not being written. It is further an objective of the present invention to precharge only bit lines adjacent to the memory column being written to minimize power requirements. It is also an object of the present invention to be able to select and activate precharge circuits using write enable and the Y decoder output.
In the present invention precharge circuits are connected to each bit line in a memory array. The precharge circuits connected to bit lines adjacent to a column being written are selected by a Y decoder selection of bit lines involved in a write operation and activated by a write enable pulse. Bit lines that are not immediately adjacent to the bit lines involved in a write operation are not precharged. Thus for any column xe2x80x9cmxe2x80x9d involved in a write operation only bit lines on either side of column xe2x80x9cmxe2x80x9d such as bit line bar BL(mxe2x88x921)B adjacent to bit line BLm, and bit line BL(m+1) adjacent to bit line bar BLmB are precharged. The bit lines are precharged to a voltage less than or equal to VDD-Vt controlled by the output voltage of a precharge control circuit depending on the strength of bit line coupling effect, where Vt is a transistor threshold voltage. Bit lines BL(mxe2x88x921), BL(m+1)B and those further away from the column being written are not precharged and in turn saving power during a write operation. The memory cells in the memory array can be either a four transistor memory cell similar to than shown in FIG. 1b, or a six transistor memory cell similar to that shown in FIG. 1a.